[agnishom]

I don't have a good answer to this question at all, since I know barely anything about this.

However, something to keep in mind is the following: signed languages are not a signed transliteration of the local language. For example, American Sign Language is not a signed way to communicate English. It has its own grammar. Therefore, when you serialize something like ASL, you do not get back something like English.

So, you have to have a different way to serialize ASL, and this is that.

[WorldMaker]

A fun way to put this in perspective as well, is that sign languages don't align exactly with spoken languages: American Sign Language and British Sign Language are very different. American Sign Language is much more closely related to French Sign Language than to British Sign Language. An ASL signer and FSL signer, despite not being able to mutually read the same language, are going to have an easier communicating than an ASL signer and a BSL signer even if they both read English. (That becomes the fallback, as I understand it, write it down in English.)

Similarly, BSL is signed throughout Britain (and I think some of the former Commonwealth?), so Welsh and Scottish "native" readers share BSL with English readers.